The purpose of this proposal is to refine magnetic resonance (MR) imaging and spectroscopy to provide a comprehensive non-invasive assessment of patients with non-alcoholic fatty liver disease (NAFLD). The specific aims are to implement and develop MR methods to qualitatively and quantitatively assess liver: (1) fat, (2) fibrosis, and (3) inflammation. Dixon variant imaging and spectroscopy will be used to assess fat. Morphological and textural analysis of images obtained with combined contrast enhancement, as well as diffusion weighted and perfusion imaging will be used to assess fibrosis. A combination of these techniques will be used to assess inflammation. Both qualitative and quantitative pathology obtained from liver biopsies will be used as the reference standard. Prospective single-center cross-sectional and longitudinal clinical studies will be performed in children and adults. Specific imaging and spectroscopic features will be correlation by histological, biochemical, and clinical findings. The proposed study is ancillary to the Non-Alcoholic Steahohepatitis Clinical Research Network, one of the obesity-related clinical trials identified by the RFA to which this proposal responds. The proposed study will be fully integrated into the University of California San Diego (UCSD) site of the Research Netowrk. UCSD is one of eight Clinical Centers in the Network. The existing clinical gold standard for diagnosis and monitoring NAFLD is biopsy. This is associated with small but definite risks and samples only a small and possibly unrepresentative region of the liver. Because of the increasing prevalence of obesity, and the risk of progression of NAFLD to steatohepatltis, cirrhosis, and hepatocellular carcinoma, the need for accurate non-invasive diagnosis is increasing. Public health statement. NAFLD is an important cause of liver disease in the United States, is becoming more and more common and affects children and adults. Developing a safe and accurate method to diagnose and follow this disease may lead to earlier diagnosis and more effective treatment.